# When going to Mars, how do you measure exact distances to determine course corrections?

When probes fly to Mars, they must adjust their trajectories multiple times during transit to ensure correct orbit insertion. They fire their small onboard rockets a little bit to do this.

But how do probes know exactly by how much to reduce their velocity and by how many degrees to deviate to ensure they hit the right position in the Martian atmosphere for precise entry? Especially since both Mars and the probes are moving very fast. And since controllers usually have a specific location on Mars they want to target.

There's obviously no GPS in space, and looking at the stars might be a good way to understand your general position in space but I doubt it would provide a precise measurement of distance down to a few hundred kilometers.

So how are precise course corrections performed in space?

• Would it be fair to say that you're asking how guidance, navigation, and control works for space probes? – Organic Marble Sep 30 '19 at 14:04
• @OrganicMarble Yes, rather true, but I'd like an answer with a focus on how the navigation is done so precisely given the large bodies and distances involved and lack of GPS. – chyeaaah Sep 30 '19 at 14:11
• If the probe is able to receive and transmit to ground stations on Earth, it is possible to send a special signal to the probe and echo it back to measure the distance and speed relative to the ground station very precisely. This method has been used for many decades since the WWII rocket V-2. – Uwe Sep 30 '19 at 14:16
• @Uwe But then how do you know the precise distance and speed of Mars? Distance is relative, right? – chyeaaah Sep 30 '19 at 14:18
• This is a good question but I think it has been answered before here. The short answer is that Earth stations monitor the spacecraft throughout the journey using precise timing of returned signals to measure distances and multiple ground stations to triangulate positions. Using those data plus precise orbital calculations (we know where all the big masses are in the solar system at all times) it's possible to calculate not only where a spacecraft is now but to approximate where it's going to be. It's a complicated process, but basically everything is done on Earth with computers and models. – uhoh Sep 30 '19 at 14:23